The Retro-Ene Reactions

The reversal of ene reactions are known as the retro-ene reactions. These reactions are favored at higher temperatures. 

The principal synthetic value of these reactions is for synthesis of allenes, dienes, and other compounds, which would be difhcult to obtain under normal conditions. Entries 1-4, illustrate the use of the retro-ene reaction to produce different classes of organic compounds [28-31]. 

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The retro-ene reaction also is of synthetic importance. While the application of high pressure facilitates the ene reaction, the retro-ene reaction is favored at higher temperatures. Furthermore small-ring strain can shift the equilibrium towards the side of the dienes. The vinylcyclopropane 11 rearranges by a synchronous process to the open-chain diene 12. Formally this process is the reverse of an intramolecular ene reaction ... 

Hydroxyalkenes are especially suitable starting materials for the retro-ene reaction since a stable carbonyl compound is then released as product. The retro-ene reaction of /3-hydroxyalkynes, e.g. 13 14, can be used for the preparation of... 

Soiling, T.I. Hammerum, S. The Retro-Ene Reaction of Gaseous Immonium Ions Revisited. J. Chem. Soc., Perkin Trans. 2 2001, 2324-2428. 

Superscript symbols are provided to facilitate an understanding of the reaction. The reverse of an ene reaction is often referred to as the retro-ene reaction. 

Reviews of the ene reactions with thiocarbonyl compounds have been published [120, 203], The sulfur retro-ene reactions are well covered in a general review on the synthetic applications of the retro-ene reaction [513],... 

The process shown in Equation 12.112, in the forward direction termed the ene reaction and in the reverse the retro-ene reaction, bears the same relationship to the [3,3]-sigmatropic reactions as does the [2,3]-elimination (p. 668) to the [2,3]-sigmatropic shift. It is also closely related to the [l,5]-rearrangement of hydrogen. The reaction can occur either intermolecularly, as in, for example, Equations 12.113 and 12.114, or intramolecularly, as in Equations 12.115 and 12.116. 

In recent years some applications of retro-ene reactions in the generation of thio-carbonyls have appeared. The retro-ene reaction is a symmetry-allowed process which resembles both [2 + 4] cycloreversion and a [1,5] sigmatropic shift of hydrogen. It usually requires high temperatures, therefore FVT has become very widespread in these reactions. 

The enophile can also be an aldehyde, ketone or imine, in which case i-hydroxy- or P-aminooIefins are obtained. These compounds may be unstable under the reaction conditions, so that at elevated temperature (>400°C) the reverse reaction takes place - the Retro-Ene Reaction. 

The mechanism and kinetic aspects of the retro-ene reaction of the allyl n-propyl sulfide and its deuterated derivatives have been studied using four different types of DFT methods with eight different levels of the basis sets.7 The mechanistic studies revealed that the reaction proceeds through an asynchronous concerted mechanism. Theoretical calculations have indicated that the reaction displays a kinetic isotope effect of 2.86 at 550.65 K. 

The retro-ene reaction of Figure 17.71 proceeds from D and directly leads to the reaction product C and the release of elemental nitrogen. 

The retro-ene reaction cleaves an unsaturated compound into two unsaturated fragments. A common example of a retro-ene reaction in organic synthesis is the acid-catalyzed decarboxylation of a (B-ketoester. The ester is hydrolyzed to the (3-ketoacid by the aqueous acid, which rapidly loses carbon dioxide to form enol. The loss of CO2 drives the reaction to the right-hand side. The enol rapidly tautomerizes to the methyl ketone (Scheme 8.17). 

The estimated relative contributions of the three paths for dodecene cracking under a variety of conditions are shown in Table V. Two trends are visible in the data. First, at constant temperature and increasing dodecene partial pressure, the relative importance of the retro-ene path decreases while that of the abstraction path increases. The contribution of the addition path remains constant. The average estimated relative contributions of the retro-ene reaction at 525°C taken from Table V correspond to a reaction order of 0.95 0.4 in dodecene this satisfies the requirement that retro-ene reaction be first order. 

The second trend is that at constant dodecene partial pressure as the temperature is increased, the relative importance of the retro-ene reaction decreases slightly, while the H abstraction path and the radical addition path show much more marked temperature dependences. The former increases while the latter decreases in relative importance. This suggests that the activation energy for hydrogen abstraction is significantly higher... 

Figure 8. Arrhenius plot for the retro-ene reaction (a) this work, 1-do-decene (b) Blades and Sandhu, 1-heptene (8) (c) Richard and Back,...

In this case, the endothermicities of Reactions A-3 and A-4 are not equal since Reaction A-3 produces an allyl ladical. Therefore, one would expect Reaction A-3 to be somewhat faster than Reaction A—4. If Radicals I and II are present in equal concentrations, then the yield of nonene from the hydrogen-abstraction path is between one and two times the yield of octene. The nonene in excess of this is formed by the retro-ene reaction. This defines a range of importance for the molecular path and by difference the contribution of the hydrogen abstraction path. This procedure is illustrated for the dodecene cracking yields from column two of Table II. 

For compounds of small molecular weight, the [1,5] shift pathway may therefore be superior in the ease of preparation, simplicity and cost. In those instances where 1,4-dienes are easily constructed with the alkene predisposed cis, the resulting conversion to vinylcyclopropanes may in fact be superior to the use of carbenoid-based reagents in those instances where the resulting vinylcyclopropanes need not be isolated (Scheme 4). The retro-ene reaction of 1,4-dienes requires temperatures which induce immediate further reactive options in the vinylcyclopropane as soon as it is generated. ... 

Some pyrolytic reactions can be seen as a reverse (retrograde) addition. Diels-Alder reaction for example is known to be reversible and retro Diels-Alder reactions are rather common. The retro-ene reaction (retro hydro-allyl addition), for example, takes place by the following mechanism ... 

In recent years this reaction has frequently been utilized for allylic functionalization and there are some interesting applications in synthesis. In connection with the topic of this chapter only the retro-ene reaction has to be taken into consideration here. 

RIpoll, J. L., Vallee, Y. Synthetic applications of the retro-ene reaction. Synthesis 1993, 659-677. 

The factors that make ene and retro-ene reactions proceed are nicely illustrated by a synthesis of enantiopure, isotopically labeled acetic acid CH(D)(T)C02H, a useful compound for studying the mechanisms of enzyme-catalyzed reactions. One ene and one retro-ene reaction occur in this synthesis. The ene reaction is driven by formation of a new tr bond at the expense of a C=C tt bond the retro-ene reaction is driven by the formation of a C=0 77 bond. Note that both pericyclic reactions proceed stereospecifically, even at the very high temperatures required for them to proceed ... 

Vinylcyclopropane-cyclopentene rearrangement (VCP-CP) reaction under thermal conditions is a useful transformation and has been extensively utilized in the synthesis of number of cyclopentanoid natural products.However, cw-alkyl-vinylcyclopropanes pose a serious problem since the retro-ene reaction, a lower energy pathway occurs readily instead. To obviate this problem, we sought to explore the excited state chemistry of c/s-alkyl-vinylcyclopropanes.t ] Thus, sensitized photolysis of (+)-A -carene readily afforded the VCP-CP product exclusively in a preparative yield. The obtention of a racemic product can be rationalized by involving a diradical intermediate.l l... 

The retro-ene reaction was used to generate 6,6-dimethyl-6-silafulvene from allylcyclo-pentadienyldimethylsilane (equation 61)215. From this reaction the dimer of the presumably first formed 6-silafulvene was isolated. Unlike other silene dimers, the isolable product does not contain a four-membered ring, although the initially formed dimer may be a 1,3-disilacyclobutane. In solution, the isolable dimer has a fluxional structure due to rapid 1,5-silyl migrations. 

While not an isomerization, the retro ene reaction of 1-pentene is the prototype for many intramolecular variants and should be discussed here. Upon heating, 1-pentene gives ethylene and propylene with log k = 11.2 — 49900/2.3Rr (Scheme 6.44). ... 

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